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I've been comparing gear ratio options. and one of the numbers i've been looking at is the difference between one gear and the next. easy way to compare these is to divide the lower ratio by the higher one. you'll notice that with almost all stock boxes, the jump is steadily increasing. each of the 4 gear ratios being about 5% closer than the last. putting a wrx .738 5th gear in with a 1.066 4th makes the 4-5 gap about 5% wider than the 3-4 than the stock .972 4th.

I just found out that the '08 WRX uses the LGT gears and runs a 4.111FD. I wonder if it is the same tranny.... more likely.

Same exact part numbers for the mainshaft, 1st and 2nd gear idlers. I know because we just installed them in a box today. Though I've always thought it was a misnomer that people call this the LGT swap. These are the same ratios that were in the v4-v6 JDM STi's and WRX sedans. They aren't new ratios, though they are new to the US market, and of course we get them with a different FD ratio than the 4.44 they see in Japan. There's a certain US vendor who has been selling these for years as a complete 5 spd kit calling it v5/v6 STi. Though, they force you to buy all 5 gears in that kit, and you end up with an additional 0.738 5th gear, which you've already got if you drive a USDM WRX. Now, with these in US cars, you are free to just buy the 3-4 gears that you need. We're building a bunch of these boxes.

However, there is one difference with the '08 WRX gears. It's a shorter 5th. They've tightened it up to a 0.780 like were used on the GC chassis 2.5RS and first generation of Outbacks.

Also, there's just a couple of minor errors in your ratio charts. The ratios you attribute to 96-99 WRX-RA/ Type RA/ 22B etc are actually 97-2000 ratios. Also, same for the WRX/STi reference directly under that in the list. Those ratios, which are the ones we are discussing above were for 97-2000 as well. Just a one year shift off of what you've got listed.

Otherwise fabulous thread. When I have more time I'll come give some of my personal input on choosing ratios and applications. Even now with all this new stuff, Subarus are still pretty restricted on what we can do with these boxes. On the Porsche gears I am making we average 12-20 different ratios for each individual gear position. At the same time, on Porsches you are generally only working with 2 final drive ratio options instead of the 3 we have with Subarus. That reduces the number of gear ratios you need to have to reach the same end. Though there's a tradeoff there and if you adjust a final drive it forces you to move all ratios the same percentage at the same time. A shorter R/P ratio that might make your 2-3-4 better might take your 1st and make in pretty much worthless.

Can anyone comment about the effect of final drive on the stress the gears see? The way I interpret it is that the higher final drive the less stress the gears will see due to mechanical advantage, but a more technical explanation would be great.

The final drive gear is exactly that, the final gear i.e it comes after all the other gears. It doesn't affect the other gears "mechanically", and is basically just a torque converter

A longer final drive (lower numerically) will be more durable than a shorter final drive in terms of the final drive breaking, but causes more radial loads due to the smaller diameter ring gear, and vice versa.

The final drive gear is exactly that, the final gear i.e it comes after all the other gears. It doesn't affect the other gears "mechanically", and is basically just a torque converter

A longer final drive (lower numerically) will be more durable than a shorter final drive in terms of the final drive breaking, but causes more radial loads due to the smaller diameter ring gear, and vice versa.

Next question: Is there a way to compare acceleration rates between different gear ratios and final drives? ie 3.166 4.11fd 1st gear in an LGT vs 3.454 3.70fd 1st gear in an 07 WRX. They result in roughly the same top speed. Obviously, power curves effect the real world results, but I assume there's an equation to give a theoretical comparison?

Thanks again! I decided to go with a jdm wrx transmission based on vehicle speed calcs and general information I've read about a shorter final drive, but numbers make things so much clearer for me. Here's the spreadsheet I put together to help make my decision: google doc The 4.44 final drive, even with longer gears, makes a substantial difference in the torque at the wheels for a given engine torque using the formula above. They only downside I see, on my stock powered car, will be cruising rpm. I figure I can just do the speed limit instead of 80 and it will be about even. It'll just take longer to get places.

Very nice write up so far. This will help many other applications where the vehicle weight and torque and tire size will be different from the suby applications. For instance the 914 midengine suby engine and tranny application that is popular. 2000 lb. midengine, 350 rwd hp 300ftlbs maybe with long street gears or close gears for the track, either with lsd.

I am thinking of rebuilting my '02 WRX transmission with STi RA gear set. I made some calculation and it seems that the ratios with the 3.90 FD will be similar to a STi 5spd 4.44FD (non-RA), except for the first gear that will be quite a bit longer.

If someone have done it before, what's the feeling of it? is the first gear too long when you start from a complete stop, or is it fine?

I am thinking of rebuilting my '02 WRX transmission with STi RA gear set. I made some calculation and it seems that the ratios with the 3.90 FD will be similar to a STi 5spd 4.44FD (non-RA), except for the first gear that will be quite a bit longer.

If someone have done it before, what's the feeling of it? is the first gear too long when you start from a complete stop, or is it fine?

Have the gears shotpeened and installed with front LSD and a 4.444FD. SO basically just swap out the 1-2 gears on the Outback NA for STi RA and put a 4.444 ring&pinion gears. This will make it Like the STi RA except with a slightly longer 3rd & 4th gear, making the shift into 5th gear not as bad of a RPM drop.

OK quick one, LGT tranny or the 4.44 fxt tranny for a DD/ rally-auto x. I heard that the FXT 5 speed, is stronger than theimpreza, but where does the LGT tranny fall in?

I'd say....find a Outback XT 5spd. Geared same as LGT but with a 4.444, which is the same as STi Version 5/6. You'd have to run a push type clutch for '06+ WRX for this application.

The LGT tranny, geared same as JDM STi Ver 5/6 with a 4.111 gives you a little more speed through the gears. The Forester XT tranny will feel shorter. The FXT tranny run the same gearing as the WRX, just with a 4.444FD compared to a 3.900FD and the 3.700FD (ewww). I'd say it was geared that way with the final drive in case folks are looking to TOW, as with the Outbacks and their shorter 5th gear on the NA models and 4.444 on the turbo models. Geared for intended application, see!

For AutoX, people will argue that short gearing is not good, then others will say too long and out of the power band. If you can SHIFT FAST, you are much quick around the autoX course with shorter ratios like the 6spd. The autoX course size matters too. Some places are SMALL, some places are HUGE. I have the JDM 6spd, and it's really nice for what I intend to do with my car. I've driven PPG.... oh that's totally geared for something else, especially running with a 3.900FD... lolz